Layered heaters are typically used in applications where space is limited, when heat output needs vary across a surface, or in ultra-clean or aggressive chemical applications. A layered heater generally comprises layers of different materials, namely, a dielectric and a resistive material, which are applied to a substrate. The dielectric material is applied first to the substrate and provides electrical isolation between the substrate and the resistive material and also minimizes current leakage during operation. The resistive material is applied to the dielectric material in a predetermined pattern and provides a resistive heater circuit. The layered heater also includes leads that connect the resistive heater circuit to a heater controller and an over-mold material that protects the lead-to-resistive circuit interface. Accordingly, layered heaters are highly customizable for a variety of heating applications.
Layered heaters may be “thick” film, “thin” film, or “thermally sprayed,” among others, wherein the primary difference between these types of layered heaters is the method in which the layers are formed. For example, the layers for thick film heaters are typically formed using processes such as screen printing, decal application, or film printing heads, among others. The layers for thin film heaters are typically formed using deposition processes such as ion plating, sputtering, chemical vapor deposition (CVD), and physical vapor deposition (PVD), among others. Yet another process distinct from thin and thick film techniques is thermal spraying, which may include by way of example flame spraying, plasma spraying, wire arc spraying, and HVOF (High Velocity Oxygen Fuel), among others.
Known systems that employ layered heaters typically include a separate temperature sensor, which is connected to the controller through another set of electrical leads in addition to the set of leads for the resistive heater circuit. The temperature sensor is often a thermocouple that is placed somewhere near the film heater and/or the process in order to provide the controller with temperature feedback for heater control. However, the thermocouple is relatively bulky, requires additional electrical leads, and fails relatively frequently. Alternately, an RTD (resistance temperature detector) may be incorporated within the layered heater as a separate layer in order to obtain more accurate temperature readings and to reduce the amount of space required as compared with a conventional thermocouple. Unfortunately, the RTD also communicates with the controller through an additional set of electrical leads. For systems that employ a large number of temperature sensors, the number of associated electrical leads for each sensor is substantial and results in added bulk and complexity to the overall heater system.
For example, one such application where electrical leads add bulk and complexity to a heater system is with injection molding systems. Injection molding systems, and more specifically hot runner systems, often include a large number of nozzles for higher cavitation molding, where multiple parts are molded in a single cycle, or shot. The nozzles are often heated to improve resin flow, and thus for each nozzle in the system, an associated set of electrical leads for a nozzle heater and a set of electrical leads for at least one temperature sensor (e.g., thermocouple) placed near the heater and/or the process must be routed from a control system to each nozzle. The routing of electrical leads is typically accomplished using an umbilical that runs from the control system to a hot runner mold system. Further, wiring channels are typically milled into plates of the mold system to route the leads to each nozzle, and therefore, an increased number of electrical leads adds cost and complexity to the hot runner mold system and adds bulk to the overall injection molding system.